Ammonium bicarbonate/ammonium carbamate activated benzyl alcohol paint stripper

ABSTRACT

Stripping action of an alkaline paint stripper is improved when ammonia is partially or completely replaced with ammonium bicarbonate or chemical equivalent thereof which decomposes into ammonia and carbon dioxide. A benzyl alcohol solvent system containing at least ammonium bicarbonate is preferably a water-in-oil emulsion emulsified with oleic hydroxyethyl imidazoline and thickened with hydroxypropyl cellulose or hydroxypropyl methylcellulose. A mixture of ammonium carbamate and ammonium bicarbonate identified as ammonium carbonate is the preferred activator for this alkaline paint stripper. Improved stripping can also be provided by reacting ammonium bicarbonate and ammonia in situ within the water-in-oil emulsion.

FIELD OF THE INVENTION

The present invention relates to alkaline paint strippers containing benzyl alcohol as a solvent. In particular the invention relates to an alkaline paint stripper containing ammonium bicarbonate/carbamate activator.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,445,939 disclosed a paint stripping and conditioning solution comprising a chlorinated solvent and a liquid alcohol such as benzyl alcohol and a wetting composition which comprises a basic metal ammonium salt of an aromatic sulfonic acid and a non-ionic surface active material. In the preparation of the metal ammonium salt the pH is adjusted to 9.8 with ammonium hydroxide.

U.S. Pat. No. 3,463,735 disclosed an aqueous cleaning composition for glass comprising both a low boiling and high boiling solvent, an alkylarylsulfonate surfactant, an alkali metal polyphosphate and ammonia.

U.S. Pat. No. 4,414,128 disclosed a composition for cleaning hard surfaces comprising a polar solvent such as benzyl alcohol, a terpene solvent, an alkylarylsulfonate surfactant, an additional solvent such as isopropanol and an alkaline material such as ammonium carbonate.

UK Patent Application GB 2 167 083 A disclosed an aqueous no-rinse hard surface cleaning composition containing a surfactant, isopropanol, benzyl alcohol, and a fugative alkaline material such as ammonia.

South African Patent Publication ZA 91/2539 entitled "IMPROVED PROCESS FOR REMOVING COATINGS FROM SENSITIVE SUBSTRATES, AND BLASTING MEDIA USEFUL THEREIN" disclosed abrading paint from aircraft with 100-500 micron water-soluble bicarbonate particles. Sodium bicarbonate is preferred but other alkali metal bicarbonates or ammonium bicarbonate are identified as being useful in combination with hydrophilic silica as the blasting media.

German Patent DD 242 419 A1 disclosed stripping old polyurethane paint from aluminum using methylene chloride, tetrahydonaphthalone, acetic butyl ester and aqueous ammonia. This alkaline formulation was intended to replace corrosive and environmentally polluting formulations containing phenol and formic acid.

German Patent DD 75 002 disclosed stripping paint from iron and non-ferrous metals using mono-chloroacetic acid, adipic acid and ammonium nitrate at temperatures of from 40° to 130° C. This acidic formulation was intended to remove scale and rust as well as paint and primer.

German Patent DE 2 142 103 disclosed stripping nickel, cobalt and copper with an alkaline bath comprising a nitrobenzyl sulfonate sodium salt and a major portion of an ingredient identified incorrectly as ammonium carbonate. Actually "ammonium carbonate" was a mixture of ammonium bicarbonate and ammonium carbamate.

Yet in spite of what was known concerning alkaline paint strippers, it was not previously known nor suggested to use substances other than ammonium nitrate or ammonia and water as activators for alkaline paint strippers.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved alkaline paint stripper wherein ammonia is partially or completely replaced by ammonium carbamate and/or ammonium bicarbonate or chemical equivalents thereof capable of releasing both ammonia and carbon dioxide.

A preferred object is to provide a water-in-oil emulsion paint stripper or coatings remover containing an effective amount of at least ammonium carbamate or ammonium bicarbonate wherein the solvent system is a benzyl alcohol phase. A preferred addition is a commercially available material identified as "Ammonium Carbonate" which in fact is a mixture of ammonium carbamate and ammonium bicarbonate. Preferred thickening agents are hydroxypropyl cellulose and hydroxypropyl methylcellulose.

DETAILED DESCRIPTION OF THE INVENTION

It was both a surprising and unexpected result to find that a benzyl alcohol solvent paint stripper gave improved stripping performance when ammonia was replaced with a commercially available material identified as "Ammonium Carbonate". However the assay of ammonia for this material was not consistent with it being a mixture of ammonium carbonate and ammonium bicarbonate. What was and is offered for sale under the name ammonium carbonate is properly identified in the literature as a mixture of ammonium carbamate and ammonium bicarbonate. It was discovered that addition of only ammonium bicarbonate to a benzyl alcohol formulation improved stripping action significantly, but the combination of ammonia and ammonium bicarbonate provided the best stripping action equivalent to use of the commercially available "Ammonium Carbonate".

Salts containing no trace of ammonium bicarbonate or ammonium carbamate or chemical equivalents thereof were found to be ineffective in improving stripping action and included: sodium bicarbonate, potassium bicarbonate, ammonium nitrate, ammonium citrate, ammonium sulfate, ammonium formate, ammonium oxalate and ammonium acetate.

Ammonium carbonate is often the word used in a patent abstract when in fact the chemical species actually used in the patent examples is ammonium bicarbonate. Ammonium bicarbonate is known to Chemistry students from the Solvay process, which may lead them to believe that ammonium carbonate is also a well known chemical.

There are three other ammonium salts with similar appearance and properties which decompose to release ammonia and carbon dioxide. In order as they appear in the Handbook of Chemistry and Physics:

    ______________________________________                                         Ammonium carbamate                                                                              NH.sub.4 NH.sub.2 CO.sub.2                                                                     78.07                                         Ammonium carbamate acid                                                                         NH.sub.4 NH.sub.2 CO.sub.2.NH.sub.4 HCO.sub.3                                                  157.13                                        carbonate                                                                      Ammonium carbonate                                                                              (NH.sub.4).sub.2 CO.sub.3.H.sub.2 O                                                            114.10                                        Ammonium bicarbonate                                                                            NH.sub.4 HCO.sub.3                                                                             79.06                                         ______________________________________                                    

As listed in the Merck Index No. 519 Ammonium Carbonate is a mixture of ammonium bicarbonate and ammonium carbamate obtained by subliming a mixture of ammonium sulfate and calcium carbonate. In Merck Index No. 518 Ammonium Carbamate or ammonium aminoformate is the "anhydride" of ammonium carbonate prepared from dry ice and liquid ammonia. The Handbook of Chemistry and Physics lists ammonium carbamate acid carbonate (synonym sal volatile) as a white crystal which similarly sublimes into carbon dioxide and ammonia. Aforementioned DE 2 142 103 recognized that in 1971 mixtures of ammonium carbamate and ammonium hydrogen carbonate (ammonium bicarbonate) were incorrectly called "ammonium carbonate."

The commercially available "Ammonium Carbonate" was determined by experimentation to be a chemical equivalent of an in situ reaction between ammonium bicarbonate and ammonia. What was originally believed to be a pH buffering effect was in fact determined to be unrelated to the influence of pH. While it is not known with any degree of certainty, it is possible that the species or combination of species responsible for improving stripping activity involve unknown intermediates or transitory complexes responsible for breaking the metal to primer bonding. U.S. Pat. No. 5,015,410 disclosed that in an acidic paint stripper an accelerator such as formic acid or acetic acid was believed to chemically attack the organic binder of the coating and thereby weaken the adhesion and cohesion of the coating.

In addition to the use of at least ammonium bicarbonate/carbamate or some other chemical equivalent such an in situ equivalent as an activator, retardation of evaporation of water as the most volatile ingredient can be achieved by dissolving or blending paraffin wax or other evaporation retardant into the continuous, organic phase of the mixed phase emulsion. The wax in the organic phase is soluble, but preferably minimally soluble. As such the amount of wax present is slightly in excess (1% by weight) of that which would saturate the organic phase; and, so as to reduce flammability, the bulk of this organic phase is less volatile than water.

There are many materials (or combinations thereof) which may serve as the organic phase of the mixed phase water-in-oil emulsion, depending on the end use, such as hydrocarbons (including petroleum distillates, both aromatic and aliphatic), alcohols, ketones, ethers, esters, and amines. For an alkaline paint stripper a preferred material as the major component of the organic phase is benzyl alcohol.

Useful additions for these water-in-oil emulsion paint stripper or coating removing formulations include, but are not limited to, oleic hydroxyethyl imidazole, sodium hydroxide, sodium silicate, 2-mercaptobenzotriazole, hydroxypropyl cellulose, hydroxypropyl methylcellulose, xanthan gum, toluene triazole, sodium polymethacrylate, mineral or paraffin oil, propylene glycol, ethoxylated nonylphenol and sodium xylene sulfonate surfactants.

While the invention has particular industrial applicability to paint stripping and coating removal of epoxy and polyurethane, it is not limited thereto and may be employed in a general sense to mixed phase alkaline emulsions.

The following procedures and examples, which are not intended to be limiting, illustrate the practice of the invention.

PROCEDURE A Process For Preparing Alkaline Paint Stripper Batch

Paint stripper batches were prepared by combining three premix batches and topping off with water. While it was possible to prepare formulations by simply mixing together all ingredients, longer mixing times and higher shear mixing was required.

First premix

In addition to at least emulsifier and benzyl alcohol a portion of aromatic hydrocarbon solvent consisting of aromatic petroleum distillate was used to pre-wet the hydroxypropyl cellulose or hydroxypropyl methylcellulose to prevent lumping. With larger mixing times or more active stirring the pre-wet step is not needed. The emulsifying agent, corrosion inhibitor(s) and other oil soluble ingredients for the first premix, which serves as the continuous organic phase, may be incorporated at this point.

Second Premix

This aqueous phase contains water and commercially available "ammonium carbonate" or ammonium bicarbonate which may be further be reacted with ammonia or a source of ammonium carbamate. This aqueous phase is mixed with the organic phase in the presence of a thickening agent or an emulsifying agent such as oleic hydroxyethyl imidazoline to produce the water-in-oil emulsion. In the emulsification process the first premix and the second premix combine to produce a dispersed aqueous phase in a continuous organic (oleaginous) phase.

Third Premix

A portion of petroleum distillate or mineral oil was used to pre-dissolve the paraffin wax. Upon adding this third premix to the mixed phase emulsion the organic phases are combined and remain separate from the aqueous phase in the form of a water-in-oil emulsion. As an alternative to adding the wax as a solution, melted wax can be added directly to the first premix with heating and mixing, thereby eliminating the need for this petroleum distillate.

EXAMPLE 1

A water base paint remover formulation was prepared according to Procedure A with the following ingredients on a % by weight basis. Approximately 0.01 g/gallon Red oil soluble dye was added to color the formulation.

    ______________________________________                                         INGREDIENT                 % BY WT.                                            ______________________________________                                         BENZYL ALCOHOL             40.00                                               AROMATIC HYDROCARBON SOLVENT                                                                              4.40                                                HYDROXYPROPYL CELLULOSE    0.45                                                BENZOTRIAZOLE              2.00                                                PARAFFIN WAX               0.20                                                AMMONIUM CARBONATE         4.00                                                WATER, DISTILLED           48.95                                               TOTAL BY WEIGHT            100.00                                              ______________________________________                                    

Portions of the paint remover were used in standard performance tests on painted metal panels and found to be fully satisfactory for paint stripping activity. A control formulation was prepared which was the same in all respects except that ammonia was added in place of ammonium carbonate which in fact was a mixture of ammonium bicarbonate and ammonium carbamate. Paint stripping action decreased even though the pH was higher (11 instead of 9) using ammonia rather than the combination of ammonium carbamate and ammonium bicarbonate.

EXAMPLE 2

A water based paint remover formulation was prepared according to Procedure A with the following ingredients on a % by weight basis.

    ______________________________________                                         INGREDIENT                % BY WT.                                             ______________________________________                                         BENZYL ALCOHOL            34.60                                                AROMATIC HYDROCARBON SOLVENT                                                                             6.80                                                 HYDROXYPROPYL CELLULOSE   0.25                                                 2-MERCAPTOBENZOTRIAZOLE   0.25                                                 OLEIC HYDROXYETHYL IMIDAZOLINE                                                                           0.70                                                 SODIUM SILICATE           0.30                                                 PARAFFIN WAX              0.20                                                 AMMONIUM BICARBONATE      6.60                                                 WATER, DISTILLED          50.30                                                TOTAL BY WEIGHT           100.00                                               ______________________________________                                    

Test samples gave improved stripping rates as compared with the water based ammoniated samples of a control formulation which contained ammonia in place of ammonium bicarbonate.

EXAMPLE 3

A water based alkaline paint remover formulation with an in situ equilibrium mixture was prepared with the following ingredients on a % by weight basis.

    ______________________________________                                         INGREDIENT                % BY WT.                                             ______________________________________                                         BENZYL ALCOHOL            37.00                                                AMMONIUM BICARBONATE      5.80                                                 AMMONIA (28%)             6.00                                                 AROMATIC HYDROCARBON SOLVENT                                                                             3.85                                                 2-MERCAPTOBENZOTRIAZOLE   0.80                                                 OLEIC HYDROXYETHYL IMIDAZOLINE                                                                           0.70                                                 HYDROXYPROPYL CELLULOSE   0.25                                                 PARAFFIN WAX              0.20                                                 WATER                     45.40                                                TOTAL BY WEIGHT           100.00                                               ______________________________________                                    

In comparison with Examples 1 and 2 this formulation with ammonium bicarbonate neutralized with an excess of ammonia in place of a commercial salt identified as "Ammonium Carbonate" showed superior stripping action for polyurethane in comparison with a formulation containing only ammonia as the activator for the benzyl alcohol solvent system.

EXAMPLE 4

Example 3 was repeated except that the ratio of ammonium bicarbonate to ammonia was varied from 90/10 to 50/50. In all cases superior paint stripping was obtained in comparison with a formulation containing only ammonia. The best stripping results involved using from 60 to 70 parts ammonium bicarbonate with from 30 to 40 parts 28% ammonia. This result is consistent with previous results which showed that ammonium bicarbonate should be the majority species for optimum paint stripping.

EXAMPLE 5

Formulations were prepared as in Examples 1 to 3 except that ammonia was replaced with either a buffering solution of alkali metal bicarbonate or ammonium salt where the pH was not allowed to be higher than 10.0 or lower than 8.0. No improved stripping activity was observed with: sodium bicarbonate, potassium bicarbonate, ammonium citrate, ammonium formate, ammonium acetate, ammonium nitrate, ammonium sulfate and ammonium oxalate. These results show that the improvement in stripping action is not a pH effect resulting from buffering, but instead is due to ammonium bicarbonate/carbamate or a chemical equivalent thereof which can decompose to release both ammonia and carbon dioxide.

EXAMPLE 6

Example 1 was repeated except that reagent grade ammonium carbamate was added instead of the commercially available "Ammonium Carbonate". Stripping action was improved relative to the use of only ammonia in the alkaline paint stripper.

EXAMPLE 7

Example 1 is repeated except that no paraffin wax evaporation retardant is added. Stripping action is not influenced, but the composition is not uniform at ambient temperatures due to evaporation.

EXAMPLE 8

The formulation of Example 1 was prepared in commercial size batches and packaged in plastic drums. The material was shipped to locations where commercial jetliners were to be stripped and repainted and used for its intended purpose. In particular on a jetliner with white paint the formulation gave a pink coloration which allowed seeing that a uniform coverage had been obtained in order to avoid missing areas during the application. The stripped paint was then conveniently removed by washing with water. Faster and more uniform stripping was obtained in relation to the use of a formulation with ammonia as the activator. In addition to improved stripping action it was observed that objectionable ammonia odor was reduced during this full scale testing. 

What is claimed is:
 1. In a paint stripping composition containing a water-in-oil emulsion which comprises (a) a continuous organic phase containing a predominant amount of benzyl alcohol, an emulsifying agent, a thickening agent and at least one corrosion inhibitor; and (b) a discrete water phase containing ammonia and a predominant amount of water wherein the improvement comprises the addition of a source of ammonia and carbon dioxide selected from the group consisting of ammonium carbamate, ammonium bicarbonate and ammonium carbonate acid carbonate and wherein benzyl alcohol is present in the composition in the composition in an amount of at least about 34% by weight of the composition and said source of ammonia and carbon dioxide is present in an amount of at least about 4% by weight of the composition.
 2. The composition of claim 1 where an oil soluble emulsifying agent is dissolved in the organic phase.
 3. The composition of claim 2 where the oil soluble emulsifying agent is oleic hydroxyethyl imidazoline.
 4. The composition of claim 1 where the thickening agent is hydroxypropyl cellulose or hydroxypropyl methylcellulose.
 5. The composition of claim 1 where ammonia and ammonium bicarbonate react in situ.
 6. The paint stripping composition of claim 1 where ammonium bicarbonate is present in excess of ammonium carbamate.
 7. The paint stripping composition of claim 1 further comprising an evaporation retardant selected from the group of paraffin wax, ceresine wax, mineral or paraffin oil, montain wax, vegetable wax, behenic acid, polyacrylate and vinyl chloride acrylic ester copolymer.
 8. The paint stripping composition of claim 7 where the evaporation retardant is paraffin wax.
 9. A method of preparing an alkaline coating remover or paint stripper water-in-oil emulsion with a predominantly benzyl alcohol organic continuous phase comprising the steps:(1) mixing an emulsifying agent, a thickening agent, or mixtures thereof into benzyl alcohol to produce an organic phase which contains a predominant amount of said benzyl alcohol; and (2) emulsifying the organic phase with an aqueous phase containing a major amount of water and a source of ammonia and carbon dioxide selected from the group consisting of ammonium carbamate, ammonium carbamate acid carbonate, and ammonium bicarbonate to produce a water-in-oil emulsion wherein benzyl alcohol is present in the emulsion in an amount of at least about 34% by weight of the emulsion and said source of ammonia and carbon dioxide is present in an amount of at least about 4% by weight of the emulsion.
 10. The method of claim 9 wherein in step (2) a mixture of ammonium carbamate and ammonium bicarbanate is added.
 11. The method of claim 9 where in step (2) a mixture of ammonium bicarbonate and ammonia is added.
 12. The method of claim 11 where the thickening agent is hydroxypropyl cellulose or hydroxypropyl methylcellulose.
 13. The method of claim 9 where the emulsifying agent is oleic hydroxyethyl imidazoline.
 14. The method of claim 13 where a corrosion inhibitor is added in step (1). 